Epac activation sensitizes rat sensory neurons through activation of Ras.

Guanine nucleotide exchange factors directly activated by cAMP (Epacs) have emerged as important signaling molecules mediating persistent hypersensitivity in animal models of inflammation, by augmenting the excitability of sensory neurons. Although Epacs activate numerous downstream signaling cascades, the intracellular signaling which mediates Epac-induced sensitization of capsaicin-sensitive sensory neurons remains unknown. Here, we demonstrate that selective activation of Epacs with 8-CPT-2'-O-Me-cAMP-AM (8CPT-AM) increases the number of action potentials (APs) generated by a ramp of depolarizing current and augments the evoked release of calcitonin gene-related peptide (CGRP) from isolated rat sensory neurons. Internal perfusion of capsaicin-sensitive sensory neurons with GDP-ßS, substituted for GTP, blocks the ability of 8CPT-AM to increase AP firing, demonstrating that Epac-induced sensitization is G-protein dependent. Treatment with 8CPT-AM activates the small G-proteins Rap1 and Ras in cultures of sensory neurons. Inhibition of Rap1, by internal perfusion of a Rap1-neutralizing antibody or through a reduction in the expression of the protein using shRNA does not alter the Epac-induced enhancement of AP generation or CGRP release, despite the fact that in most other cell types, Epacs act as Rap-GEFs. In contrast, inhibition of Ras through expression of a dominant negative Ras (DN-Ras) or through internal perfusion of a Ras-neutralizing antibody blocks the increase in AP firing and attenuates the increase in the evoked release of CGRP induced by Epac activation. Thus, in this subpopulation of nociceptive sensory neurons, it is the novel interplay between Epacs and Ras, rather than the canonical Epacs and Rap1 pathway, that is critical for mediating Epac-induced sensitization.

The development and maintenance of paclitaxel-induced neuropathic pain require activation of the sphingosine 1-phosphate receptor subtype 1.

The ceramide-sphingosine 1-phosphate (S1P) rheostat is important in regulating cell fate. Several chemotherapeutic agents, including paclitaxel (Taxol), involve pro-apoptotic ceramide in their anticancer effects. The ceramide-to-S1P pathway is also implicated in the development of pain, raising the intriguing possibility that these sphingolipids may contribute to chemotherapy- induced painful peripheral neuropathy, which can be a critical dose-limiting side effect of many widely used chemotherapeutic agents.We demonstrate that the development of paclitaxel-induced neuropathic pain was associated with ceramide and S1P formation in the spinal dorsal horn that corresponded with the engagement of S1P receptor subtype 1 (S1PR(1))- dependent neuroinflammatory processes as follows: activation of redox-sensitive transcription factors (NFκB) and MAPKs (ERK and p38) as well as enhanced formation of pro-inflammatory and neuroexcitatory cytokines (TNF-α and IL-1ß). Intrathecal delivery of the S1PR1 antagonist W146 reduced these neuroinflammatory processes but increased IL-10 and IL-4, potent anti-inflammatory/ neuroprotective cytokines. Additionally, spinal W146 reversed established neuropathic pain. Noteworthy, systemic administration of the S1PR1 modulator FTY720 (Food and Drug Administration- approved for multiple sclerosis) attenuated the activation of these neuroinflammatory processes and abrogated neuropathic pain without altering anticancer properties of paclitaxel and with beneficial effects extended to oxaliplatin. Similar effects were observed with other structurally and chemically unrelated S1PR1 modulators (ponesimod and CYM-5442) and S1PR1 antagonists (NIBR-14/15) but not S1PR1 agonists (SEW2871). Our findings identify for the first time the S1P/S1PR1 axis as a promising molecular and therapeutic target in chemotherapy-induced painful peripheral neuropathy, establish a mechanistic insight into the biomolecular signaling pathways, and provide the rationale for the clinical evaluation of FTY720 in chronic pain patients.

Sphingosine 1-phosphate enhances the excitability of rat sensory neurons through activation of sphingosine 1-phosphate receptors 1 and/or 3.

BACKGROUND: Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that acts through a family of five G-protein-coupled receptors (S1PR1-5) and plays a key role in regulating the inflammatory response. Our previous studies demonstrated that rat sensory neurons express the mRNAs for all five S1PRs and that S1P increases neuronal excitability primarily, but not exclusively, through S1PR1. This raises the question as to which other S1PRs mediate the enhanced excitability. METHODS: Isolated sensory neurons were treated with either short-interfering RNAs (siRNAs) or a variety of pharmacological agents targeted to S1PR1/R2/R3 to determine the role(s) of these receptors in regulating neuronal excitability. The excitability of isolated sensory neurons was assessed by using whole-cell patch-clamp recording to measure the capacity of these cells to fire action potentials (APs). RESULTS: After siRNA treatment, exposure to S1P failed to augment the excitability. Pooled siRNA targeted to S1PR1 and R3 also blocked the enhanced excitability produced by S1P. Consistent with the siRNA results, pretreatment with W146 and CAY10444, selective antagonists for S1PR1 and S1PR3, respectively, prevented the S1P-induced increase in neuronal excitability. Similarly, S1P failed to augment excitability after pretreatment with either VPC 23019, which is a S1PR1 and R3 antagonist, or VPC 44116, the phosphonate analog of VPC 23019. Acute exposure (10 to 15 min) to either of the well-established functional antagonists, FTY720 or CYM-5442, produced a significant increase in the excitability. Moreover, after a 1-h pretreatment with FTY720 (an agonist for S1PR1/R3/R4/R5), neither SEW2871 (S1PR1 selective agonist) nor S1P augmented the excitability. However, after pretreatment with CYM-5442 (selective for S1PR1), SEW2871 was ineffective, but S1P increased the excitability of some, but not all, sensory neurons. CONCLUSIONS: These results demonstrate that the enhanced excitability produced by S1P is mediated by activation of S1PR1 and/or S1PR3.

Reduced expression of SynGAP, a neuronal GTPase-activating protein, enhances capsaicin-induced peripheral sensitization.

Synaptic GTPase-activating protein (SynGAP) is a neuronal-specific Ras/Rap-GAP that increases the hydrolysis rate of GTP to GDP, converting Ras/Rap from the active into the inactive form. The Ras protein family modulates a wide range of cellular pathways including those involved in sensitization of sensory neurons. Since GAPs regulate Ras activity, SynGAP might be an important regulator of peripheral sensitization and pain. Therefore, we evaluated excitability, stimulus-evoked release of the neuropeptide calcitonin gene-related peptide (CGRP), and nociception from wild-type (WT) mice and those with a heterozygous mutation of the SynGAP gene (SynGAP(+/-)). Our results demonstrate that SynGAP is expressed in primary afferent sensory neurons and that the capsaicin-stimulated CGRP release from spinal cord slices was two-fold higher from SynGAP(+/-) mice than that observed from WT mouse tissue, consistent with an increase in expression of the capsaicin receptor, transient receptor potential cation channel subfamily V member 1 (TRPV1), in SynGAP(+/-) dorsal root ganglia. However, there was no difference between the two genotypes in potassium-stimulated release of CGRP, the number of action potentials generated by a ramp of depolarizing current, or mechanical hypernociception elicited by intraplantar injection of capsaicin. In contrast, capsaicin-induced thermal hypernociception occurred at lower doses of capsaicin and had a longer duration in SynGAP(+/-) mice than WT mice. These results provide the first evidence that SynGAP is an important regulator of neuropeptide release from primary sensory neurons and can modulate capsaicin-induced hypernociception, demonstrating the importance of GAP regulation in signaling pathways that play a role in peripheral sensitization.

Regulation of N-type voltage-gated calcium channels (Cav2.2) and transmitter release by collapsin response mediator protein-2 (CRMP-2) in sensory neurons.

Collapsin response mediator proteins (CRMPs) mediate signal transduction of neurite outgrowth and axonal guidance during neuronal development. Voltage-gated Ca(2+) channels and interacting proteins are essential in neuronal signaling and synaptic transmission during this period. We recently identified the presynaptic N-type voltage-gated Ca(2+) channel (Cav2.2) as a CRMP-2-interacting partner. Here, we investigated the effects of a functional association of CRMP-2 with Cav2.2 in sensory neurons. Cav2.2 colocalized with CRMP-2 at immature synapses and growth cones, in mature synapses and in cell bodies of dorsal root ganglion (DRG) neurons. Co-immunoprecipitation experiments showed that CRMP-2 associates with Cav2.2 from DRG lysates. Overexpression of CRMP-2 fused to enhanced green fluorescent protein (EGFP) in DRG neurons, via nucleofection, resulted in a significant increase in Cav2.2 current density compared with cells expressing EGFP. CRMP-2 manipulation changed the surface levels of Cav2.2. Because CRMP-2 is localized to synaptophysin-positive puncta in dense DRG cultures, we tested whether this CRMP-2-mediated alteration of Ca(2+) currents culminated in changes in synaptic transmission. Following a brief high-K(+)-induced stimulation, these puncta became loaded with FM4-64 dye. In EGFP and neurons expressing CRMP-2-EGFP, similar densities of FM-loaded puncta were observed. Finally, CRMP-2 overexpression in DRG increased release of the immunoreactive neurotransmitter calcitonin gene-related peptide (iCGRP) by approximately 70%, whereas siRNA targeting CRMP-2 significantly reduced release of iCGRP by approximately 54% compared with control cultures. These findings support a novel role for CRMP-2 in the regulation of N-type Ca(2+) channels and in transmitter release.

Nerve growth factor, sphingomyelins, and sensitization in sensory neurons.

Because nerve growth factor (NGF) is elevated during inflammation, plays a causal role in the initiation of hyperalgesia, and is known to activate the sphingomyelin signalling pathway, we examined whether NGF and its putative second messenger, ceramide, could modulate the excitability of capsaicin-sensitive adult sensory neurons. Using the whole-cell patch-clamp recording technique, exposure of isolated sensory neurons to either 100 ng/mL NGF or 1 mmol/L N-acetyl sphingosine (C2-ceramide) produced a 3-4 fold increase in the number of action potentials (APs) evoked by a ramp of depolarizing current in a time-dependent manner. Intracellular perfusion with bacterial sphingomyelinase (SMase) also increased the number of APs suggesting that the release of native ceramide enhanced neuronal excitability. Glutathione, an inhibitor of neutral SMase, completely blocked the NGF-induced augmentation of AP firing, whereas dithiothreitol, an inhibitor of acidic SMase, was without effect. In the presence of glutathione and NGF, exogenous ceramide still enhanced the number of evoked APs, indicating that the sensitizing action of ceramide was downstream of NGF. To investigate the mechanisms of actions for NGF and ceramide, isolated membrane currents were examined. Both NGF and ceramide facilitated the peak amplitude of the TTX-resistant sodium current (TTX-R I(Na)) by approximately 1.5-fold and shifted the activation to more hyperpolarized voltages. In addition, NGF and ceramide suppressed an outward potassium current (I(K)) by ~35%. The inflammatory prostaglandin, PGE2, produced an additional suppression of I(K) after exposure to ceramide (~35%), suggesting that these agents might act on different targets. Based on the existing literature, it is not clear whether this NGF-induced sensitization is mediated by the high-affinity TrkA receptor or the low-affinity p75 neurotrophin receptor. Pretreatment with the p75 blocking antibody completely prevents the NGF-induced increase in the number of APs evoked by the current ramp. Although the sensitization by NGF was blocked, the antibody had no effect on the capacity of ceramide, a putative downstream signalling molecule, to enhance the excitability. Ceramide can be metabolized by ceramidase to sphingosine (Sph) and Sph to sphingosine 1-phosphate (S1P) by sphingosine kinase. It is well established that each of these products of sphingomyelin metabolism can act as intracellular signalling molecules. This raises the question as to whether the enhanced excitability produced by NGF was mediated directly by ceramide or required additional metabolism to Sph and/or S1P. Sph applied externally did not affect the neuronal excitability whereas internally perfused Sph augmented the number of APs evoked by the depolarizing ramp. Furthermore, internally perfused S1P enhanced the number of evoked APs. This sensitizing action of NGF, ceramide, and internally perfused Sph, were abolished by dimethylsphingosine (DMS), an inhibitor of sphingosine kinase. In contrast, internally perfused S1P enhanced the number of evoked APs in the presence of DMS. These observations support the idea that the metabolism of ceramide/Sph to S1P is critical for the sphingolipid-induced modulation of excitability. Thus, our findings indicate that the pro-inflammatory agent, NGF, can rapidly enhance the excitability of sensory neurons. This NGF-induced sensitization is mediated by activation of the sphingomyelin signalling pathway wherein intracellular S1P derived from ceramide, acts as an internal second messenger to regulate membrane excitability, however, the effector system whereby S1P modulates excitability remains undetermined.

Peripheral Synthesis of an Atypical Protein Kinase C Mediates the Enhancement of Excitability and the Development of Mechanical Hyperalgesia Produced by Nerve Growth Factor.

Nerve growth factor (NGF) plays a key role in the initiation as well as the prolonged heightened pain sensitivity of the inflammatory response. Previously, we showed that NGF rapidly augmented both the excitability of isolated rat sensory neurons and the mechanical sensitivity of the rat's hind paw. The increase in excitability and sensitivity was blocked by the myristoylated pseudosubstrate inhibitor of atypical PKCs (mPSI), suggesting that an atypical PKC may play a key regulatory role in generating this heightened sensitivity. Our findings raised the question as to whether NGF directs changes in translational control, as suggested for long-lasting long-term potentiation (LTP), or whether NGF leads to the activation of an atypical PKC by other mechanisms. The current studies demonstrate that enhanced action potential (AP) firing produced by NGF was blocked by inhibitors of translation, but not transcription. In parallel, in vitro studies showed that NGF elevated the protein levels of PKMζ, which was also prevented by inhibitors of translation. Intraplantar injection of NGF in the rat hind paw produced a rapid and maintained increase in mechanical sensitivity whose onset was delayed by translation inhibitors. Established NGF-induced hypersensitivity could be transiently reversed by injection of rapamycin or mPSI. These results suggest that NGF produces a rapid increase in the synthesis of PKMζ protein in the paw that augments neuronal sensitivity and that the ongoing translational expression of PKMζ plays a critical role in generating as well as maintaining the heightened sensitivity produced by NGF.

The TrkA receptor mediates experimental thermal hyperalgesia produced by nerve growth factor: Modulation by the p75 neurotrophin receptor.

The p75 neurotrophin receptor (p75NTR) and its activation of the sphingomyelin signaling cascade are essential for mechanical hypersensitivity resulting from locally injected nerve growth factor (NGF). Here the roles of the same effectors, and of the tropomyosin receptor kinase A (TrkA) receptor, are evaluated for thermal hyperalgesia from NGF. Sensitivity of rat hind paw plantar skin to thermal stimulation after local sub-cutaneous injection of NGF (500ng) was measured by the latency for paw withdrawal (PWL) from a radiant heat source. PWL was reduced from baseline values at 0.5-22h by ∼40% from that in naïve or vehicle-injected rats, and recovered to pre-injection levels by 48h. Local pre-injection with a p75NTR blocking antibody did not affect the acute thermal hyperalgesia (0.5-3.5h) but hastened its recovery so that it had reversed to baseline by 22h. In addition, GW4869 (2mM), an inhibitor of the neutral sphingomyelinase (nSMase) that is an enzyme in the p75NTR pathway, also failed to prevent thermal hyperalgesia. However, C2-ceramide, an analog of the ceramide produced by sphingomyelinase, did cause thermal hyperalgesia. Injection of an anti-TrkA antibody known to promote dimerization and activation of that receptor, independent of NGF, also caused thermal hyperalgesia, and prevented the further reduction of PWL from subsequently injected NGF. A non-specific inhibitor of tropomyosin receptor kinases, K252a, prevented thermal hyperalgesia from NGF, but not that from the anti-TrkA antibody. These findings suggest that the TrkA receptor has a predominant role in thermal hypersensitivity induced by NGF, while p75NTR and its pathway intermediates serve a modulatory role.

ET--phone the pain clinic.

Recent findings by Khodorova et al. demonstrate that the vasoconstrictor endothelin-1 plays an important role in certain nociceptive behaviors in an animal model of pain, through activation of sensory neurons. Endothelin-1 might also have the unexpected capacity to release an opioid from surrounding keratinocytes and thereby inhibit the pain response. Such results suggest that, in the periphery, there are important interactions between sensory nerve terminals and surrounding cells, and that glia and keratinocytes could modulate the perception of environmental stimuli to a greater extent than previously considered.

Tumor necrosis factor alpha and interleukin-1beta stimulate the expression of cyclooxygenase II but do not alter prostaglandin E2 receptor mRNA levels in cultured dorsal root ganglia cells.

Tumor necrosis factor alpha (TNFalpha) and interleukin 1beta (IL-1beta) are pro-inflammatory cytokines capable of altering the sensitivity of sensory neurons. Because sensitization elicited by IL-1beta and TNFalpha is blocked by inhibition of the inducible enzyme, cyclooxygenase-II (COX-2), we examined whether these cytokines could increase COX-2 expression in dorsal root ganglion (DRG) cultures. Treatment of cell cultures with either IL-1beta or TNFalpha increases immunoreactive COX-2, as measured by immunoblotting, in a time- and concentration-dependent manner. A 24-h pretreatment with 10 ng/ml IL-1beta or 50 ng/ml TNFalpha augmented COX-2 expression 50- and 8-fold over basal levels, respectively. Immunohistochemistry established the presence of COX-2-like immunoreactivity in both neuronal and non-neuronal cells in culture. The addition of IL-1 receptor antagonist blocked the induction of COX-2 expression by IL-1beta, but did not alter TNFalpha-stimulated increases in COX-2, indicating that the mechanism of TNFalpha is not limited to increasing the expression of IL-1beta. The basal and TNFalpha-induced expression of COX-2 was not dependent on the presence of NGF in the growth media. IL-1beta and TNFalpha treatment for 24 h enhanced prostaglandin E2 (PGE2) production 2-4-fold, which was blocked by pretreatment with the COX-2 inhibitor, NS-398. Exposing cultures to PGE2, IL-1beta, or TNFalpha for 24 h did not alter PGE2 receptor (EP) mRNA levels. These results indicate that TNFalpha and IL-1beta induce the functional expression of COX-2 but not EP receptors in DRG cells in culture and suggest that cytokine-induced sensitization of sensory neurons is secondary to prostaglandin production and not alterations in EP receptors.

Synaptic and membrane properties of neurons in the dorsomedial hypothalamus.

Studies in intact rats have shown that the dorsomedial hypothalamus (DMH) plays a key role in generating stress-induced physiologic changes, including activation of the hypothalamic-pituitary-adrenal axis through direct projections to paraventricular hypothalamic nucleus (PVN). However, little is known about the cellular properties of DMH neurons. We employed whole-cell patch-clamp recording techniques to characterize membrane properties and spontaneous post-synaptic currents (PSCs) in DMH neurons, including those projecting to PVN (identified by prior injection of DiI into PVN), in rat hypothalamic slices. DMH neurons (n=86 total) had uniform membrane properties. However, PVN-projecting neurons (n=32) had higher action potential (AP) thresholds, and fired fewer APs in response to current injection. Spontaneous PVN-projecting neurons (n=20) also fired APs at lower rates (4.8+/-0.6 Hz) than spontaneous neurons of unknown projection (n=38; 7.3+/-1.1 Hz). Spontaneous PSCs were observed in all neurons: One population expressed rapid decay characteristics (1.5-2.0 ms) and was blocked by non-NMDA ionotropic glutamate receptor antagonists NBQX or CNQX. Remaining PSCs reversed near E(Cl), were blocked by the GABA(A) receptor antagonists picrotoxin or bicuculline methiodide (BMI), and had longer decay time constants (4.5-6.0 ms) that were modulated by pentobarbital. Tetrodotoxin markedly reduced the frequency of PSCs sensitive to NBQX but not to BMI. Thus, DMH is made up of electrophysiologically similar neurons and PVN-projecting neurons are less excitable than neurons of unknown projection. Furthermore, as suggested by studies in intact rats, neurons in the DMH, including those projecting to the PVN, are regulated by tonic GABA(A) and non-NMDA glutamate receptor-mediated synaptic transmission.

NGF-mediated sensitization of the excitability of rat sensory neurons is prevented by a blocking antibody to the p75 neurotrophin receptor.

Nerve growth factor (NGF) can play a causal role in the initiation of hyperalgesia. Recent work demonstrates that NGF can act directly on nociceptive sensory neurons to augment their sensitivity to a variety of stimuli. Based on the existing literature, it is not clear whether this sensitization is mediated by the high-affinity TrkA receptor or the low-affinity p75 neurotrophin receptor. We examined whether a blocking antibody to the p75 neurotrophin receptor can prevent the NGF-induced enhancement of excitability in capsaicin-sensitive small-diameter sensory neurons that have been isolated from the adult rat. In this report, pretreatment with the p75 blocking antibody completely prevents the NGF-induced increase in the number of action potentials evoked by a ramp of depolarizing current as well as the suppression of a delayed rectifier-type of potassium current(s) in these neurons. Although the sensitization by NGF was blocked, the antibody had no effect on the capacity of ceramide, a putative downstream signaling molecule, to either enhance the excitability or inhibit the potassium current. These results indicate that NGF can increase the excitability of nociceptive sensory neurons through activation of the p75 neurotrophin receptor and its consequent liberation of ceramide from neuronal sphingomyelins.

Nerve growth factor mediates a switch in intracellular signaling for PGE2-induced sensitization of sensory neurons from protein kinase A to Epac.

We examined whether nerve growth factor (NGF), an inflammatory mediator that contributes to chronic hypersensitivity, alters the intracellular signaling that mediates the sensitizing actions of PGE2 from activation of protein kinase A (PKA) to exchange proteins directly activated by cAMP (Epacs). When isolated sensory neurons are grown in the absence of added NGF, but not in cultures grown with 30 ng/ml NGF, inhibiting protein kinase A (PKA) activity blocks the ability of PGE2 to augment capsaicin-evoked release of the neuropeptide CGRP and to increase the number of action potentials (APs) evoked by a ramp of current. Growing sensory neurons in culture in the presence of increasing concentrations of NGF increases the expression of Epac2, but not Epac1. An intradermal injection of complete Freund's adjuvant into the rat hindpaw also increases the expression of Epac2, but not Epac1 in the dorsal root ganglia and spinal cord: an effect blocked by intraplantar administration of NGF antibodies. Treating cultures grown in the presence of 30 ng/ml NGF with Epac1siRNA significantly reduced the expression of Epac1, but not Epac2, and did not block the ability of PGE2 to augment capsaicin-evoked release of CGRP from sensory neurons. Exposing neuronal cultures grown in NGF to Epac2siRNAreduced the expression of Epac2, but not Epac1 and prevented the PGE2-induced augmentation of capsaicin and potassium-evoked CGRP release in sensory neurons and the PGE2-induced increase in the number of APs generated by a ramp of current. In neurons grown with no added NGF, Epac siRNAs did not attenuate PGE2-induced sensitization. These results demonstrate that NGF, through increasing Epac2 expression, alters the signaling cascade that mediates PGE2-induced sensitization of sensory neurons, thus providing a novel mechanism for maintaining PGE2-induced hypersensitivity during inflammation.

Therapeutic targeting of the ceramide-to-sphingosine 1-phosphate pathway in pain.

Approximately 20% of the population in Western countries suffers from chronic pain syndromes for which treatments are frequently insufficient or non-existent. In particular, chronic pain management with opiate/narcotic analgesics is often hampered by the development of analgesic tolerance and hyperalgesia, necessitating escalating doses to achieve pain relief. There is a major need for renewed focus on novel targets that will be effective in both neuropathic and inflammatory pain. Compelling evidence implicates ceramide-to-sphingosine 1-phosphate (S1P) pathways as contributors to pain of diverse etiologies. Moreover, S1P and its receptors are emerging as important neuronal and immune cell regulators interacting at several sites in the pain pathway. It is therefore timely and important to critically evaluate the pharmacological basis for targeting the ceramide-to-S1P pathway as an approach to pain management.

Knockdown of the sphingosine-1-phosphate receptor S1PR1 reduces pain behaviors induced by local inflammation of the rat sensory ganglion.

Sphingosine 1-phosphate (S1P) is a key immune mediator regulating migration of immune cells to sites of inflammation. S1P actions are mediated by a family of five G protein-coupled receptors. Sensory neurons express many of these receptors, and in vitro S1P has excitatory effects on small-diameter sensory neurons, many mediated by the S1P receptor 1 (S1PR1). This study investigated the role of S1P in regulating the sensitivity of DRG neurons. We found that in vivo perfusion of the normal L5 DRG with S1P increased mechanical sensitivity. Microelectrode recordings in isolated whole ganglia showed that large- and medium-diameter cells, as well as small-diameter cells, increased firing in the presence of S1P. To further determine the role of S1PRs, we examined the effects of in vivo S1PR1 knockdown in the L4 and L5 sensory ganglia. Small interfering RNA directed against S1PR1 did not affect baseline mechanical sensitivity in normal animals, in which S1P levels are expected to be low. However, when the L5 ganglion was locally inflamed, a procedure that leads to rapid and sustained mechanical hypersensitivity, S1PR1 siRNA injected animals showed significantly less hypersensitivity than animals injected with scrambled siRNA. Reduced expression of S1PR1, but not S1PR2 or S1PR3, was confirmed with qPCR methods. The results indicate that the S1PR1 receptors in sensory ganglia cells may play an important role in regulating behavioral sensitivity during inflammation.

Direct effects of HIV-1 Tat on excitability and survival of primary dorsal root ganglion neurons: possible contribution to HIV-1-associated pain.

The vast majority of people living with human immunodeficiency virus type 1 (HIV-1) have pain syndrome, which has a significant impact on their quality of life. The underlying causes of HIV-1-associated pain are not likely attributable to direct viral infection of the nervous system due to the lack of evidence of neuronal infection by HIV-1. However, HIV-1 proteins are possibly involved as they have been implicated in neuronal damage and death. The current study assesses the direct effects of HIV-1 Tat, one of potent neurotoxic viral proteins released from HIV-1-infected cells, on the excitability and survival of rat primary dorsal root ganglion (DRG) neurons. We demonstrated that HIV-1 Tat triggered rapid and sustained enhancement of the excitability of small-diameter rat primary DRG neurons, which was accompanied by marked reductions in the rheobase and resting membrane potential (RMP), and an increase in the resistance at threshold (R(Th)). Such Tat-induced DRG hyperexcitability may be a consequence of the inhibition of cyclin-dependent kinase 5 (Cdk5) activity. Tat rapidly inhibited Cdk5 kinase activity and mRNA production, and roscovitine, a well-known Cdk5 inhibitor, induced a very similar pattern of DRG hyperexcitability. Indeed, pre-application of Tat prevented roscovitine from having additional effects on the RMP and action potentials (APs) of DRGs. However, Tat-mediated actions on the rheobase and R(Th) were accelerated by roscovitine. These results suggest that Tat-mediated changes in DRG excitability are partly facilitated by Cdk5 inhibition. In addition, Cdk5 is most abundant in DRG neurons and participates in the regulation of pain signaling. We also demonstrated that HIV-1 Tat markedly induced apoptosis of primary DRG neurons after exposure for longer than 48 h. Together, this work indicates that HIV-1 proteins are capable of producing pain signaling through direct actions on excitability and survival of sensory neurons.

Role of calcium in volume-activated chloride currents in a mouse cholangiocyte cell line.

Volume-activated Cl(-) channels (VACCs) play vital roles in many cells including cholangiocytes. Previously, we characterized the VACCs in mouse cholangiocytes. Since calcium plays an important role in VACC regulation in many cells, we have studied the effect of calcium modulation on the regulatory volume decrease (RVD) and VACC currents in mouse bile duct cells (MBDCs). Cell volume measurements were assessed by a Coulter counter with cell sizer, and conventional whole-cell patch-clamp techniques were used to study the role of calcium on RVD and VACC currents. Cell volume study indicated that MBDCs exhibited RVD, which was inhibited by 5-nitro-2'-(3-phenylpropylamino)-benzoate (NPPB), 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetoxymethyl ester (BAPTA-AM) but not by removal of extracellular calcium. During hypotonic challenge, MBDCs exhibited an outwardly rectified current, which was significantly inhibited by administration of classical chloride channel inhibitors such as NPPB and tamoxifen. Chelation of the intracellular calcium with BAPTA-AM or removal of extracellular calcium and calcium channel blocker had no significant effect on VACC currents during hypotonic challenge. In addition to VACC, MBDC had a calcium-activated chloride channel, which was inhibited by NPPB. The present study is the first to systemically study the role of calcium on the VACC and RVD in mouse cholangiocytes and demonstrates that a certain level of intracellular calcium is necessary for RVD but the activation of VACC during RVD does not require calcium. These findings suggest that calcium does not have a direct regulatory role on VACC but has a permissive role on RVD in cholangiocytes.

Unraveling the story of NGF-mediated sensitization of nociceptive sensory neurons: ON or OFF the Trks?

Nerve Growth Factor (NGF) is produced by and affects a number of immune and inflammatory cells. As part of the inflammatory response, NGF directly or indirectly alters the sensitivity of small diameter sensory neurons that communicate noxious information. The question remains as to the receptors and intracellular signaling cascades that mediate this sensitizing action of NGF. Although the general consensus is that NGF produces peripheral sensitization by activating TrkA, recent work suggests that p75 also contributes. Thus, both NGF receptors appear to contribute to peripheral sensitization although whether they act independently or together remains to be determined. Furthermore, controversy exists as to the downstream signaling pathways involved in NGF-induced peripheral sensitization.

Electrophysiological characterization of volume-activated chloride currents in mouse cholangiocyte cell line.

Recent electrophysiological and radioisotope efflux studies have demonstrated various Cl(-) channels in cholangiocytes including volume-activated Cl(-) channels (VACC). Because VACCs play prominent roles in many vital cellular functions and physiology in cholangiocytes, we have examined their electrophysiological characteristics in mouse cholangiocytes to provide an important framework for studying in the future. The present study is to characterize VACCs expressed in the mouse bile duct cell (MBDC) line, conditionally immortalized by SV40 virus. Conventional whole cell patch-clamp techniques were used to study the electrophysiological characteristics of VACC in MBDC. When the MBDCs were exposed to hypotonic solution, they exhibited an outwardly rectified current, which was significantly inhibited by replacing chloride in the bath solution with gluconate or glutamate and by administration of classic chloride channel inhibitors 5-nitro-2-(3-phenylpropylamino)-benzoate, glybenclamide, DIDS, and tamoxifen. These inhibitory effects were reversible with washing them out from the bath solution. Moreover, the ion selectivity of the volume-activated channel to different anions indicates that it is more permeable to SCN(-) > I(-) >/= Cl(-) > F(-) >/= acetate >/= glutamate >/= gluconate. These electrophysiological characteristics demonstrate that the volume-activated current observed is a VACC. In addition, the VACC in MBDC has electrophysiological characteristics similar to those of the VACC in human cholangiocarcinoma cell line. The present study is the first to characterize the VACC in mouse cholangiocyte and will provide an important framework for further studies to examine and understand the role of the VACC in biliary secretion and ion-transport physiology.

Activation of epidermal vanilloid receptor-1 induces release of proinflammatory mediators in human keratinocytes.

During dermal injury and the associated trauma a number of compounds are released that can mediate the inflammatory response. Determining the cellular mechanisms that initiate the inflammatory responses to acute keratinocyte damage is important for understanding the regulation of epidermal inflammation. The recently cloned vanilloid receptor-1 (VR1) is a polymodal receptor, responding to thermal, pH, or vanilloids such as capsaicin stimulation. Although VR1 has been localized only on sensory neurons and within the central nervous system, recent evidence suggests a functional VR1 is expressed in human skin and epidermal cells. Using reverse transcription-polymerase chain reaction and immunoblotting we report that human keratinocytes and the human keratinocyte cell line HaCaT express VR1. Consistent with neuronal VR1, activation of epidermal VR1 by capsaicin induced a calcium influx. Treating HaCaT cells with capsaicin resulted in a dose-dependent expression of cyclooxygenase-2 (COX-2), whereas pretreatment with the VR1 receptor antagonist capsazepine abolished the capsaicin-stimulated increase in COX-2 expression. Furthermore, the capsaicin-induced expression of COX-2 was dependent on extracellular calcium. Activation of the epidermal VR1 by capsaicin also resulted in an increased release of interleukin-8 and prostaglandin E2, and the stimulated release was attenuated by capsazepine. The finding that VR1 is expressed by keratinocytes is of great importance because it expands the putative role of VR1 beyond that of pain perception. Our results suggest that VR1 expression in keratinocytes may have a role in the inflammation that occurs secondary to epidermal damage or insult, and thus may function as a sensor for noxious cutaneous stimulation.